This invention relates to a method of laser casting, for example, a copper-based material. In particular, the invention presents a method to directly melt pure copper (Cu) powder with the help of other elements (X) such as, for example, (nickel (Ni), iron (Fe) or tungsten (W)) using CO2 laser. Using this technique, Cu alloys (Cu+X) and composites Cuxe2x88x92Y and Cuxe2x88x92Xxe2x88x92Y (Y=tungsten carbide (WC), titanium carbide (TiC), titanium (Ti) and graphite (C)) can be synthesised from elemental powder mixtures which are prepared by mechanical mixing or milling processes. The developed laser casting process may advantageously be used to fabricate complex three-dimensional objects, by multi-layer overlapping, which may be used in electrical discharging machining (EDM) electrodes, rapid die and mould tooling, or other system components.
The method and apparatus of selective laser sintering (SLS) are described in U.S. patents such as U.S. Pat. No. 4,863,538 (1989), U.S. Pat. Nos. 4,938,816 and 4,944,817 (1990), U.S. Pat. No. 5,076,869 (1991) and U.S. Pat. No. 5,182,170 (1993). In SLS, parts are built by selective sintering or local melting of a binder in a thin layer of powder particles using a CO2 laser beam. The interaction of the laser beam with the powder raises the temperature to the melting point of the powder binder, resulting in particle bonding, fusing the particles to one another and to the previous layer. After an additional layer of powder is deposited via a roller mechanism on top of the sintered layer, the succeeding layer is similarly sintered and built directly on top of it. In this way, the entire solid can be built layer by layer. Each layer of the building process consists of the required cross-section of the part at a given height. The unsintered powder in each layer remains in the powder bed during processing to support overhangs and other structures in subsequent layers. The completed part is revealed by brushing off the loose powder surrounding it and the unsintered powder can then be reused. Despite of the capability of the SLS to build parts of various materials, post-processing, such as debinder and Cu infiltration, is often needed to achieve working strength. Shrinkage of the built part after the debinder and infiltration process results in distortion.
A selective metal powder sintering process was described by Van der Schueren and Druth in xe2x80x9cPowder deposition in selective metal powder sinteringxe2x80x9d in Rapid Prototyping Journal, Vol. 1, Number 3, 1995, pp23-31. In this process particles in a Fexe2x80x94Cu powder mixture were selectively bound by means of liquid phase sintering initiated by a Nd-YAG laser beam. The powder deposition mainly depended on the powder propertiesxe2x80x94in this case on the individual Cu or Fe powder propertiesxe2x80x94and resulted in compromises on the powder mixtures as well as in modifications of the deposition mechanism.
EOSINT M system, as described in U.S. Pat. Nos. 5,753,274, 5,730,925, 5,658,412, was the first commercial system for direct laser sintering of metallic powder. The word xe2x80x9cdirectxe2x80x9d implies that the material constituents are directly laser sintered to produce a high density part requiring little or no post-processing. A related patent on parts formed by direct sintering is U.S. Pat. No. 5,732,323 which describes processing of powders based on an iron-group metal. Currently, the only metallic material that is available commercially for direct metal sintering is a bronze-nickel alloy by Electrolux and a newly developed metal powder M Cu 3201 by EOS. Direct selective laser sintering involves directly melting and consolidating selected regions of a powder bed to form a desired shape having high or full density. Direct metal laser sintering involves melting the component matrix and obtaining the appropriate amount of flow from the molten material. The appropriate amount of flow is critical and can be described as the flow that eliminates porosity, produces a highly dense part and maintains tight dimensional tolerances. The appropriate amount of flow is controlled by factors such as atmosphere, powder bed temperature and laser"" energy density. Three important parameters governing the energy density are laser power, scan spacing and scan speed2:
An=P/xcexdxcex4(J/cm2)xe2x80x83xe2x80x83(1)
where An is the energy density; P is the incident laser power (Watts); xcexd is the laser scan speed (cm/s); and xcex4 is the scan spacing (cm).
If the energy density is too high, the surface begins to vaporize before a significant depth of molten material is produced. The sintered layer thickness decreases with increasing scan speed due to the shorter interaction (sintering) time. This thickness also decreases with decreasing scan line spacing if the laser beam spot is larger than the spacing. More scan overlapping will occur with smaller scan line spacing. The thermal conductivity and reflectivity of the sintered solid are higher than those of the powder. When more scan overlapping occurs, more laser energy will be transferred away by heat conduction through the sintered solid and reflected away by the sintered solid surface resulting in a decrease in the layer thickness.
The amount of light energy of the laser beam absorbed by a metallic surface is proportional to 1-R, where R is its reflectivity. The reflectivity of a material is defined as the ratio of the radiant power reflected to the radiant power incident on the surface. It indicates the fraction of the incident light that is absorbed and contributes to heating effects, and is most dependent on the electrical conductivity. A metal with high electrical conductivity has high reflectivity, for example, copper and nickel. High-density energy is required to sinter a material with high reflectivity, such as Cu. Another important characteristic is thermal diffusibility. A material with high thermal diffusibility will normally allow a greater depth of fusion penetration with no thermal shock or cracking.
At the CO2 laser wavelength of 10.6 xcexcm, where R is close to unity, 1-R becomes very small. High-density energy is thus required to sinter a material like copper. The difference in the value of R becomes important at long wavelengths. For copper at 10.6 xcexcm, 1-R is about 0.02, whereas for steel it is about 0.05. As steel absorbs 2.5 times as much of the incident light as copper, it is easier to melt steel with a CO2 laser than metals such as aluminum or copper. Attempts to coat the powder surface to improve heat absorption or reduce reflection are not always effective because of poor thermal coupling between the coating and the powder. The reflectivity problem has been a barrier to the application of CO2 lasers to the melting of metals such as copper or gold.
According to one aspect of the invention there is provided a method of laser casting a metal-based alloy or composite comprising:
milling elemental metal powder having a relatively high reflectivity at the wavelength of the laser with at least one other material which absorbs laser energy more readily than said elemental metal powder to form said metal-based mixture; and
laser casting said metal-based mixture; wherein said milling is conducted for a period sufficient to form at least a partial coating of said at least one material on particles of said elemental metal powder.
More particularly the invention provides a method of laser casting a copper-based alloy or composite comprising:
milling elemental copper powder with at least one other material which absorbs laser energy more readily than elemental copper powder to form said copper-based mixture; and
laser casting said copper-based alloy or composite by application of a laser to said copper-based mixture; wherein said milling is conducted for a period sufficient to form at least a partial coating of said at least one material on particles of said elemental copper powder.
FIG. 1a illustrates a process of laser casting in building strips;
FIG. 1b illustrates the process of laser casting in a multi-layered part.
FIG. 2 shows an x-ray diffraction (XRD) results of Cu-Ni powder before and after the laser casting process.
FIG. 3a shows the microstructure of the laser cast Cu-Ni material.
FIG. 3b shows a laser-cast tracking having smooth surface morphology.
FIG. 4 shows the x-ray (XRD) diffraction spectra of a Cu-WC(Ni) system before and after laser casting.
FIG. 5a shows the microstructure of the laser cast Cu-6.7% WC composite;
FIG. 5b is a photomicrograph showing the segregation of WC particles;
FIG. 5c shows the cross-section of a multi-layered part with CU 19.13% Ni 7.5% WC.
FIG. 6 shows an x-ray diffraction (CRD) spectrum of a CU-Ti-C(Ni) system comparing mixture of powders to after laser casting.
FIG. 7a shows the dense microstructure of the Cu-Ti-C-Ni system.
FIG. 7b is another view of the microstructure of the Cu-Ti-C-Ni system.
FIG. 8 shows the x-ray diffraction (XRD) spectrum of laser cast Cu 10% Fe.
The following descrip tion of the invention will be limited to Cu based systems. However, it will be recognised that the principle of the invention may also be applied to other metal systems where the laser casting of a metal having high reflectivity is required.
Cu is a very versatile and common material for applications requiring good electrical and thermal conductivities, especially for use as a base material in EDM electrodes. However, Cu is not easily melted by CO2 laser due to its high reflectivity to the laser. The present invention in one aspect advantageously provides a method of melting Cu using CO2 laser with the help of other element(s) to form Cu alloys and Cu-based in-situ composites.
Elemental Cu powder is initially milled with other elemental powder or compounds which absorb laser energy more readily. To achieve a certain degree of coating of the second element on the Cu particles, the powder mixture is preferably ball milled for about 1 to 2 hours under the protection of Argon gas. The mills may be planetary ball mills, attritors and horizontal mills. Relatively high energy of milling is used. The coating on the Cu particles advantageously enhances the conduction of heat to the adjacent Cu particles. 0 to 3% by weight of process controlling agent (PCA), which can be, for example, stearic acid or other low melting organics and flux, may be used to prevent or minimize cold welding. If graphite is used, no PCA should be introduced in the milling. In general, minimal PCA should be incorporated into the powder mixture to reduce contamination.
Besides material compositions which will be discussed hereafter, the properties and quality of the part may be altered by altering the processing parameters, such as laser power output, beam spot size and its scanning speed. As such, these parameters are advantageously controlled in order to achieve optimal performance. In forming the particular material system and building the multi-layered parts, the processing parameters are preferably controlled to provide a laser power of 50-1000 W, beam spot size of 0.2-5.0 mm and laser scanning speed of 100-1500 mm/min.
In the laser casting process, inert gas such as Argon, is used to prevent oxidation of materials to be cast. When Ti is used, inert gas protection becomes more important. A reduction atmosphere of CO may also be used.
All powder systems are preferably mechanically mixed for at least about one hour followed by ball milling for at least about one to two hours to achieve at least partial coating of Ni, W, Fe, Ti, TiC or WC on the Cu particles. The ball mill machine is preferably run at a speed of 150-300 rpm using a ball size of 15-30 mm diameter with weight ratio of ball-to-particle-size of from 5-20:1 for a duration of from 1 to 4 hours. Up to 3% by weight of process controlling agent (PCA) may be added in the ball milling to prevent excessive cold welding. If C is used during milling, PCA should not be added to reduce surface contamination during milling.
Laser scanning speed preferably spans from 100 to 1500 mm/min using 50-1000 W laser power. Beam spot size generally ranges from 0.1-5.0 mm. These parameters should provide sufficient heat energy to melt the Cu component well. However, to build a fine layer part, the spot size can be further reduced and scanning speed increased. Inert gas of Argon is advantageously used during the laser scan. Due to the high reactivity of Ti with oxygen, an Argon chamber with purity of at least 99% is preferably used.
To obtain optimal parameters for a particular powder mixture, a stainless steel plate of approximately 5 mm thickness may be placed on a flat surface as a substrate. A 1 mm thick stainless steel plate with a central cut out is advantageously placed on top of a thicker plate. Metal powder is then preferably laid in the frame and lightly compressed and flattened by a roller in order to form a uniform powder bed. The laser beam is then programmed to scan the powder bed in horizontal strips, with each strip formed at a different laser scanning speed.
FIG. 1 illustrates the process of laser casting in building strips (FIG. 1a) and a multi-layered part (FIG. 1b). After the first layer of powder is scanned, the xe2x80x9cpistonxe2x80x9d like part is lowered down and another layer of powder is spread on top of the preceding layer of powder. The uneven powder is slightly compressed and levelled using the levelling roller. The new layer of powder is then scanned by the laser. This process is repeated until a three-dimensional part is built.
In the laser scanning process of a Cuxe2x80x94Ni system, chemical homogenisation between the Cu and Ni occurs to form a homogeneous melt. The new homogeneous solid phase thus formed is Cuxe2x80x94Ni. The amount of Ni in the solid phase is preferably at least 5%. Even using a low percentage of Ni, the powder may still be laser-cast to form a dense part. A study of microstructure of the Nixe2x80x94Cu system reveals a dendritic structure. It has been determined that as the amount of Ni is decreased from 57.5% to 9.59%, the dendritic microstructure progressively disappears.
In Cuxe2x80x94Tixe2x80x94C, Cuxe2x80x94Ni/Tixe2x80x94C systems, Cu and a mixture of Cu and Ni are used as the matrix while Ti and C react with each other to form in-situ TiC as reinforcement. The formation of fine hard TiC particles increases the strength, Young""s modulus and wear resistance of the part produced. The maximum volume percentage of TiC is generally 50%.
In Cuxe2x80x94Ni/TiC systems, TiC is incorporated into a Cuxe2x80x94Ni matrix by laser casting the Cuxe2x80x94Ni together with TiC particles. It has been observed that the TiC is uniformly distributed in the Cuxe2x80x94Ni solid solution. The resulting microstructure shows good interface between the reinforcement phase and the matrix phase.
Cu/WC and Cuxe2x80x94Ni/WC composite parts may be synthesised using Cu/WC, Cuxe2x80x94Ni/WC systems. Cu can be melted with or without the help of Ni. WC may also be used to facilitate heat absorption and the melting of Cu. Since WC is much heavier than Cu and Cuxe2x80x94Ni solution, WC particles usually settle down to the bottom of the laser scanned line or strip. To minimize or prevent inhomogeneity in this case, thin laser scan lines are advantageously used.
Cuxe2x80x94Fe systems are also available using the present method. In this case at least 10% of Fe is preferable used for successful laser casting. More Fe may enhance formability of parts but will decrease electrical and thermal conductivities. Depending on applications, the amount of Fe may be varied from 10% to 50%.
When considering a Cuxe2x80x94W (Ni) system, about 10% W is preferably used to ensure successful laser casting. Since Cu and W are not solutable, Cu may be pushed to the two sides of the laser cast melt line. A thin laser cast line may help to reduce inhomogeneity of distribution of Cu in this case. Addition of Ni advantageously increases the wetting between the three constituents.